Information systems for handling numerous document and data formats are quickly becoming open systems where different devices are tied to one another to provide solutions to customers' needs. A key feature in such open systems is enabling electronic documents to be printed on a range of output devices. In order to achieve device-independence, efficient methods of image enhancement are required. Hence, raster conversion technology, where a bitmap created for a first output device, for example a bitmap containing half-bitting, is altered so as to be adequately rendered by a second output device, has become an important aspect of the open system technology.
Distortion in bitmap images rendered by digital printing techniques is a consequence of the low resolution of the bitmap or the low sampling rates of the analog image that is intended to be represented. In the past, limitations on the capabilities of xerographic printing systems, for example the low modulation transfer function (MTF) of such systems, reduced the distortion to an acceptable level. At the same time, the approach commonly employed to reduce distortion was to increase the spatial resolution of the bitmap image and use half-bitting techniques, so as to adequately render the detail lost at lower resolutions. Unfortunately, with improved imaging and development processes, xerographic printing systems now render half-bitted and jaggy images with such clarity as to be objectionable to persons demanding high quality printed output. Thus, the half-bitting techniques that once enabled the reasonable rendition of a low resolution image or character font are now a detriment to accurately reproducing the finest detail of the analog image across a range of printers. The present invention enables device-independent printing by using image processing to enhance binary input images having a first resolution so as to produce images having a second resolution; either binary images with a spatial resolution greater that the first resolution, or multiple-bit per pixel images at the first resolution, so that the distortions within the input image are reduced or eliminated upon printing.
Heretofore, resolution enhancement has been accomplished using various techniques, including those described in the following disclosures which may be relevant:
U.S. Pat. No. 4,544,264 (issued Oct. 1, 1985) and U.S. Pat. No. 4,625,222 (issued Nov. 26, 1986), both issued to Bassetti et al. describe enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scan lines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. No. 4,544,922 to Watanabe et al., issued Oct. 1, 1985, teaches a smoothing circuit for an orthogonal matrix display. The circuit adds or removes a "small dot" on the display from either the first or last third of a dot clock (DCK) period that is one-third the period in which a standard dot of the original pattern is displayed.
U.S. Pat. No. 4,933,689 to Yoknis, issued Jun. 12, 1990, describes a method for enhancing a displayed image in a laser exposed dot matrix format to produce softened edge contours using three pulses, a central pulse plus leading and trailing enhancement pulses that are separated therefrom. The purpose of the leading and trailing pulses is to create a blurred or grayed region at the leading and trailing edges of each associated character.
U.S. Pat. No. 5,150,311 to Long et al., issued Sep. 22, 1992, discloses a system for producing print-dot data suitable for driving a hardcopy printing device. More specifically, the print-dot data is selectively obtained from a conversion operation carried out by a matrix and dot generator combination which respectively generate subtractive color components and a pattern of high resolution print-dots therefrom.
U.S. Pat. No. 5,161,035 to Muramatsu, issued Nov. 3, 1992, teaches an image processing device for enlarging an original bitmap image, where the image density distribution is maintained.
U.S. Pat. No. 5,193,008 to Frazier et al., issued Mar. 9, 1993, further describes the resolution enhancement apparatus as one that includes the ability to rasterize the image to be printed at twice the resolution of the printer. The printer then outputs the higher resolution image using an interleaving technique that generates developable dots between scan lines by energizing corresponding dots on adjacent scan lines at a level which will not be developed, but where the overlapping portion of the two corresponding dots will be developable.
U.S. Pat. No. 5,206,741 to Shimura et al., issued Apr. 27, 1993, discloses an image processing apparatus for processing image data to be output by a printing unit. A conversion unit converts pixel image data within an image memory into data having a resolution equal to the output resolution of the print mechanism.
A number of the previously described patents and publications are summarized in Torrey Pines Research, Behind Hewlett-Packard's Patent on Resolution Enhancement.TM. Technology, (Becky Colgan ed., BIS CAP International, 1990) pp. 1-60, including concepts associated with resolution enhancement.
James C. Stoffel et al. in A Survey of Electronic Techniques for Pictorial Image Reproduction, IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981, incorporated by reference for its teachings, discloses image processing algorithms that can be used to transform continuous tone and halftone pictorial image input into spatially encoded representations compatible with binary output processes. A set of image quality and processing complexity metrics are also defined so as to evaluate a number of image processing algorithms with respect to their ability to reproduce continuous tone or halftone pictorial input.
L. Steidel in Technology Overview: Resolution Enhancement Technologies for Laser Printers, LaserMaster Corp., discusses three currently available implementations for vertical resolution enhancement; Resolution Enhancement Technology, Paired Scan Line Scheme, and TurboRes. In all cases, the horizontal resolution of the laser scanner is increased by increasing the clock speed. On the other hand, the vertical resolution is enhanced by combining the weaker laser laser energy from a brief laser flash, which leaves only residual or fringe energy on the image drum at the periphery of a pixel of an adjacent pixel on a second scan line.
Of particular importance are the following patents and publications:
U.S. Pat. No. 4,437,122 to Walsh et al., issued Mar. 13, 1984, teaches an improved method of converting low resolution images into images of higher resolution for printing so as to simultaneously increase density and smooth character edges. In a CRT display or hardcopy output apparatus, the invention is accomplished by converting an original pixel into a higher resolution 3.times.3 enhanced representation. The status of each of the nine elements in the enhanced representation is determined as a result of an examination of the neighboring pixels of the original pixel.
U.S. Pat. No. 4,841,375 to Nakajima et al., issued Jun. 20, 1989, discloses an image resolution conversion apparatus that converts image data having a predetermined pixel density to a pixel density matching that of a printer so as to enable printing by the printer. The pixel density converter includes: a conversion-pixel position detector for detecting the position of a converted pixel; an original-pixel extractor for extracting a reference original pixel; a conversion-pixel density operation circuit for calculating the density of a conversion pixel; a threshold-value setter for dynamically setting a threshold value; a binary encoding circuit for digitizing the conversion-image density; an input interface for inputting image data; an output interface for outputting image data; and a control circuit for controlling the input/output (I/O) and the conversion operations.
U.S. Pat. No. 4,847,641 (issued Jul. 11, 1989) and U.S. Pat. No. 5,005,139 (issued Apr. 2, 1991) to Tung disclose print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. patent application Ser. No. 07/513,415, and the corresponding Japanese laid-open patent publication 4-227584 published Aug. 17, 1992, to Mailloux et al. disclose a method to enable the conversion of binary image data originally generated at a lower resolution into representative binary image data of a higher resolution, wherein the conversion ratio, or magnification factor, is an integer value. Included within the resolution magnification invention are methods for smoothing the interpolated output image and thereby reducing objectionable visual characteristics observable in digitally encoded data using conventional magnification techniques.
Mathematical Morphology in Image Processing, pp. 43-90 (Edward R. Dougherty ed., Marcel Dekker 1992), hereby incorporated by reference, describes efficient design strategies for an optimal binary digital morphological filter. A suboptimal design methodology is investigated for binary filters in order to facilitate a computationally manageable design.
Robert P. Loce et al., in Optimal Morphological Restoration: The Morphological Filter Mean-Absolute-Error Theorem, Journal of Visual Communications and Image Representation, (Academic Press), Vol. 3, No. 4, December 1992, pp. 412-432, hereby incorporated by reference, teach expressions for the mean-absolute restoration error of general morphological filters formed from erosion bases in terms of mean-absolute errors of single-erosion filters. In the binary setting, the expansion is a union of erosions, while in the gray-scale setting the expansion is a maxima of erosions. Expressing the mean-absolute-error theorem in a recursive form leads to a unified methodology for the design of optimal (suboptimal) morphological restoration filters. Applications to binary-image, gray-scale signal, and order-statistic restoration on images are included.
Edward R. Dougherty et al., in Optimal mean-absolute-error hit-or-miss filters: morphological representation and estimation of the binary conditional expectation, Optical Engineering, Vol. 32, No. 4, April 1993, pp. 815-827, incorporated herein by reference, disclose the use of a hit-or-miss operator as a building block for optimal binary restoration filters. Filter design methodologies are given for general-, maximum-, and minimum-noise environments and for iterative filters.
It is an object of the present invention to improve the appearance of binary images that may contain half-bitted regions therein, and more particularly to identify regions of a first binary image where it is desirable to smooth jagged or curved edges or where half-bitting is present, converting those regions to a multiple-bit per pixel representation, and generating an enhanced multiple-bit per pixel representation of the region in a second image having the same resolution as the first image or in a second binary image with a resolution greater than the first image so as to enable an improved rendering of the first binary image by printing of the second image.
In accordance with the present invention, there is provided a method for increasing the resolution of a binary input image to be printed by an electronic printing system, comprising the steps of: storing a portion of the binary input image in a scanline buffer memory; selecting a central pixel within the binary input image; determining the binary state of the central pixel and of a plurality of pixels neighboring the central pixel; comparing the binary pixel states of the central pixel and the neighboring pixels to associated pixel states in a predefined template; detecting when a match occurs between the central and neighboring pixel states and the predefined template; defining, upon detection of a match, a multi-bit digital signal as a value associated with the predefined template identified by the match; and converting the multi-bit digital signal into a resolution enhanced output pixel suitable for printing by the electronic printing system.
In accordance with another aspect of the present invention, there is provided a method for improving the appearance of a binary input image to be printed by an electronic printing system, the binary input image containing a half-bitted region therein, comprising the steps of: storing a portion of the binary input image in scanline buffer memory; identifying the region of half-bitted image pixels within the binary input image; generating a plurality of multi-bit digital signals to represent image pixels within half-bitted region; and converting each multi-bit digital signal into a resolution enhanced output pixel suitable for printing by the electronic printing system.
In accordance with yet another aspect of the present invention, there is provided an apparatus for increasing the resolution of a binary input image to be printed by an electronic printing system, including: scanline buffer memory for storing a portion of the binary input image; target pixel selection means, operating on the stored portion of the binary input image, for selecting a central pixel within the binary input image; template matching means for comparing the binary pixel state of the central pixel and a plurality of pixels neighboring the central pixel to a predetermined template pattern so as to determine a multi-bit digital signal which represents a desired output density for the central pixel; and a video signal generator for converting the multi-bit digital signal into at least one output pixel suitable for printing by the electronic printing system.